When filling the fuel tank of a vehicle with petrol vapour tends to escape from the tank filler neck to atmosphere. However, it is now recognised that petrol vapour includes benzine and that this is a carcinogenic material. Clearly, it is unacceptable to allow the uncontrolled release of dangerous materials into the environment. In order to prevent this fuel dispensers are now increasingly provided with vapour recovery systems. In the U.S.A. in particular the provision of fuel dispensers with vapour recovery systems is expected to be made mandatory.
Fuel is customarily delivered to the tank through a nozzle via a fuel hose and vapours are recovered from the immediate vicinity of the nozzle through a manifold with inlets in it which surrounds the nozzle. The manifold is connected to a vapour recovery line which conveys the vapour to the main fuel reservoir from whence the fuel was drawn or a separate underground tank. In one known vapour recovery system, the vapours and any fuel emerging from the tank being filled are drawn through the manifold into the vapour recovery line by a vapour recovery pump. Ideally a 1:1 ratio of fuel dispensed to vapour recovered must be achieved in order to ensure efficient vapour removal and to avoid pressurising the tank/reservoir to which the fuel vapour is returned. In order to ensure that this ratio is maintained the flow of recovered fuel vapour must be controlled.
In one known system described in U.S. Pat. No. 5,040,577 the volumetric flow of a vapour recovery means is controlled by a programmed microprocessor. Electrical signals are derived from sensors that are related in a known way to the volumetric flow of the fuel dispenser and are then applied to the microprocessor. The microprocessor then determines on the basis of information stored therein the parameters of an electrical signal that can be applied to the vapour recovery means in order to achieve the required vapour recovery rate. The volumetric vapour flow can be controlled by adjusting the speed of the motor driving the vapour recovery pump and/or by controlling the position of a variable valve or damper in the vapour recovery line.
Whereas the volumetric flow rate in the vapour recovery line may be set to equal that in the fuel delivery hose, there are conditions, such as differences in the temperature of the fuel in the vehicle tank and fuel from the fuel supply reservoir under which it is desirable to use a volumetric vapour flow rate that is different from the volumetric fuel flow rate. To this end it is desirable to obtain an indication of the volumetric vapour flow rate. Any differences between the measured vapour flow rate and the vapour flow rate required to match the fuel flow rate can then be compensated for adjusting the speed of the vapour recovery pump and/or the position of the variable valve or damper situated in the vapour recovery line.
In one embodiment, a sensor generates an electrical signal corresponding to the hydraulic pressure at the inlet side of the pump for the vapour recovery means. Under average conditions, the pressure will have a desired nominal value. When it is less than this value, the nominal pressure is restored by decreasing the volumetric flow of the vapour recovery means, and when it is greater than this value, nominal pressure is restored by increasing the volumetric flow of the vapour recovery means. The microprocessor is programmed to respond to the signal representing the pressure and provide signals for controlling the volumetric flow of the vapour recovery means. This is particularly easy to do if, in accordance with this invention, the motor driving the recovery pump is of the stepping type because it is driven at a speed determined by the repetition rate of drive pulses, and this can be easily changed.
The closed loop system described hereinabove gives relatively good system accuracy and can compensate for wear in the system, but the sensors for measuring the vapour flow rate, in particular, have problems associated with them.
One known sensor for use in measuring the fuel vapour flow rate in a fuel vapour recovery line is the so-called "turbine" type. Essentially this comprises a rotary member having radially extending spokes projecting from a central hub. Each of the spokes carries a vane. The transducer is placed in the fuel vapour recovery line in such a way as to be rotated by the passage of vapour past the vanes. The speed of rotation of the rotary member determines the vapour flow rate past it.
This type of sensor is relatively inexpensive, but is not ideally suited to this type of application as it does not cope well with liquid or liquid/vapour phases which may occasionally present themselves. Moreover, it is slow to respond which can give rise to false signals during delay times.
Another known sensor for this type of application takes the form of a thermal sensor chip. As vapour passes over the surface of the chip it has the effect of cooling it. The amount of cooling is determined by the chip and is indicative of the vapour flow rate past it.
The principal disadvantage associated with this type of sensor is that it is relatively expensive. Moreover, because the chip is very delicate it is not usually placed directly in the fuel vapour recovery line, but rather in a bypass loop. In the bypass loop the sensor only measures a portion of the actual fuel vapour flow and therefore it cannot be relied upon to be completely accurate. Furthermore, this type of sensor does not work well when liquid fuel is drawn in with the vapour. Not only can the sensor output vary, but it is difficult to clear this condition.
Yet another known sensor for this type of application is a variable orifice sensor. This takes the form of a ball or float mounted within a tapered tube mounted vertically in the wall of the fuel vapour recovery line. As the flow increases, then the float will lift in the tube to allow sufficient orifice for the passage of gas. The degree of displacement is indicative of the vapour flow rate within the line.
The float movement is then sensed externally (usually by a magnet) and this is then converted into an analogue signal. Here again problems arise when slugs of liquid fuel try to pass the float. Then the float is ejected upwards to its maximum position which could damage the device.
Another sensor is the fixed orifice plate with measuring equipment at the inlet/outlet positions. This type of sensor usually has a small orifice in order to obtain reasonable values of pressures. This means the sensor is very restrictive on high flows due to its nature.